As the speed of digital electronic circuits has increased, certain physical effects that could previously be ignored have become more significant. One such effect is known as "undershoot". Undershoot is a sensitivity to negative voltage transients appearing at an input to the device. Undershoot is caused by an excessive positive current drain from outputs which are not logically connected to the input subjected to the transient. As expected, a negative voltage spike appearing at an input to a device will cause a voltage fluctuation or glitch at an output logically connected thereto. However, under certain conditions, the negative voltage spike will draw current from other parts of the device such as collector regions which can create glitches in outputs not logically connected to the input pin having the spike.
A typical circuit that might exhibit an undershoot sensitivity is a programmable array logic (PAL) device. A PAL will normally have a plurality of input pins and a plurality of output pins, some of which output and input pins are not logically connected. A functional undershoot failure occurs when a negative voltage spike appears on an input pin and one or more output pins which are not logically connected to that input pin glitch. Such a glitch may be characterized by an ephemeral or indefinite change in logic level appearing on the pin in question.
The ability of an electronic device to absorb negative transient spikes is known as undershoot tolerance or resistance. At present there does not appear to be adequate methods for analyzing and/or testing electronic devices for undershoot resistance.